In the conventional Rockwell tester as used for measuring the hardness of materials, the tester is first loaded against the workpiece under a minor load. A reading of the depth of indentation is taken under the minor load; and the tester is then loaded under a major load. After release of the major load, a new reading is taken for comparison with the first reading. Using the regular Rockwell scales, the hardness of the material can thus be determined. With workpieces of substantial thickness, the Rockwell tester is conventionally loaded to a 10 kg. minor load.
When measuring the described of materials which are quite thin, the same procedure descirbed above is used. However, the usual minor load to which the tester is loaded is only 3 kg. and Rockwell superficial scales are used to take a reading of the hardness of the material. Most often, separate testers are required for measuring materials where different Rockwell scales are used. This is because the testers are constructed with internal spring mechanisms for loading at one specified minor load.
Multiple purpose testers have been constructed which are capable of applying either 3 or 10 kg. minor loads. Testers of this type use the weight of the lever arm structure of the tester for applying a 3 kg. minor load, while the 10 kg. minor load is applied by adding external weights. The major problem with this type of system lies in the fact that should any force impede the action of the lever arm, the minor load will be invalid. For this reason, means must be included for applying the major loads without disturbing the lever arm. This, in turn, requires considerable alteration of a standard tester in converting it to one capable of applying different minor loads.